Sizing agents are used in the paper industry to provide water repellency to the paper and board. The need for water repellency is twofold: either to impart water resistance for the end use of the paper or to provide a production advantage to the papermaker. The first is exemplified by a liquid container or a printed sheet. Both require a degree of water repellency to provide their function in the marketplace. The productivity aspect is determined by the need for a controlled uptake of an aqueous liquid added subsequent to the web forming process. Examples are size presses (commonly used on printing and writing papers as well as many board grades), coaters both on and off machine, and calender boxes. If the aqueous pick up of the subsequent coating is excessive the paper machine will have to slow down in order to dry the additional water. Also, the coating will not be held on the surface if there is insufficient sizing and surface properties may suffer.
Inorganic pigments are widely used as fillers in the paper industry. This use is primarily in printing and writing grades where they provide improved optical and printing properties. They also reduce overall costs since most pigments cost less than cellulose pulp. Inorganic fillers also reduce the paper strength so their use is limited to that amount that will produce the needed optical properties without overall degradation of strength. The papermaker is continually striving to increase the level of mineral filler to reduce costs but to maintain good runnability on his machine to increase productivity. If the paper becomes too weak, it may break in process under the stresses and strains of the process. Additionally, the use of increased fillers has a detrimental effect on sizing. As the filler loading is increased the sizing level decreases sometimes rather dramatically. In the sense that sizing is needed to adequately run the paper machine, any upset in sizing will lead to an upset in productivity. The difficulty in sizing high filler loadings is thought to be due to the increased surface area of the fillers relative to the pulp. Since internal sizes work by increasing the contact angle of the surface to be sized, the addition of more surface area causes an increased demand for size. This is especially true of cellulose reactive sizes which are much more efficient than the traditional rosin sizes. An additional problem is the difficulty of retaining both size and filler in highly filled systems. As the level of filler increases, it is more difficult to retain the filler in the web so the filler retention usually decreases. In order to prevent this, retention aids are commonly employed that aid filler retention. As the filler level increases, the need for internal size will also increase.
The object of the present invention is to allow the papermaker to add a wide range of filler loading without seriously affecting the sizing level in the sheet. This would greatly simplify the manufacturing problems of the papermaker who could now concentrate on achieving the optimum balance of strength properties and cost without concern that the sizing level would be adversely affected. In this way, both paper properties and productivity could be maximized independent of each other.
There is much prior art regarding ways to improve the sizability of the filler. Generally, they fall into two categories: (1) use of cationic treatments to improve the retention of filler, and (2) treatment with sizing agents as a method to retain the size.
The use of cationic treatments is a widely used tactic in paper chemical additives, since the pulp fiber is anionic in nature. Cationic additives are attracted to the cellulose pulp and retention of the additive may be improved. This is not so effective with filler particles which are large and thus, not strongly bound by the relatively weak attractive forces of opposite charge attraction. A cationic polymer is smaller and will be relatively, tightly held by the same degree of attraction. Another factor is the fact that most fillers are retained as agglomerated clusters by an entrapment mechanism that occurs by filtration during web formation rather than by individual particles being adsorbed on the cellulose surface.
The other prior art category relates to the use of fillers as carriers of a sizing agent, in particular, ketene dimer (KD). The technology for using KD to size paper had been delayed by the inability to make stable, easily retained dispersions for use by the paper mill. Absorption of KD onto filler particles which were then retained by a filtration or flocculation filtration mechanism was an early method mentioned in the prior art. The effectiveness of these methods was found to be much lower than the now conventional wet end addition of cationic emulsion. In addition, the level of sizing agent was coupled to the level of filler addition so that it was impossible to increase sizing without increasing the filler level and this wasn't usually desired.
Neither of these methodologies, however, addresses the problem we described above; namely, the large loss in sizing efficiency with added levels of filler.